1. Field of the Invention
The present invention relates to photopolymerizable compositions and more specifically to photopolymerizable compositions containing a novel photopolymerization initiator.
2. Description of the Prior Art
Hitherto, it has been widely known that unsaturated compounds capable of hardening upon exposure to actinic radiation are used as components for compositions such as coatings, printing inks or adhesives, etc., and, when electromagnetic waves such as visible light, ultraviolet radiation or X-rays, etc., or corpuscular rays such as electron beams, neutron beams or .alpha.-rays are applied to such compositions, the above-described compounds harden the compositions by polymerization and if such operation is carried out in the presence of a polymerization initiator the polymerization rate remarkably increases. For example, this is described in U.S. Pat. Nos. 3,551,235, 3,551,246, 3,551,311 and 3,558,387, Belgian Pat. No. 808,179 and Japanese Patent Application (OPI) No. 110781/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") (corresponding to U.S. patent application Ser. No. 328,442, filed Jan. 31, 1973) now abandon. Though products obtained by these methods have excellent flexibility, chemical resistance, abrasion resistance, luster, adhesive property and color hue, images having good quality cannot be reproduced for minute images when the operation is carried out with a slight vibration because the compositions have low sensitivity to hardening and a long period of time is required for image exposure. Furthermore, films of the compositions are easily deformed or deteriorated by the large quantity of heat generated when the radiation dosage is increased to make up for the low sensitivity of the composition.
As the result of investigations directed to increasing the sensitivity of photopolymerizable compositions, it has been found that photopolymerization initiators having a certain composition remarkably increase the photopolymerization velocity of polymerizable compounds having ethylenically unsaturated double bonds.
Though it has been described in Eur. Polymer J. by G. A. Delzenne, U. Laridon and H. Peeters, 6, 933 (1970) that O-acyloximes such as 1-phenyl-1,2-propanediol-2-O-benzoyloxime are effective photopolymerization initiators and in Chemistry Letters by M. Yoshida, H. Sakuragi, T. Nishimura, S. Ishikawa and K. Tokumaru, 1125 (1975) that the decomposition of O-acyloximes is accelerated by ketones, they do not have the sensitivity necessary for practical use. It has been found that when cyclic isoxazolones or isoxazinones are used as O-acyl ketones and combined with various other initiators, a peculiar high sensitivity is obtained.